In a typical chemical pulp mill, such as a Kraft pulp mill, white liquor containing active cooking chemicals, sodium hydroxide (NaOH) and sodium sulphide (Na2S) is used for cooking wood chips. The residue, called black liquor, is concentrated by evaporation and burned in a recovery boiler furnace to yield an inorganic smelt of sodium carbonate (Na2CO3) and sodium sulphide. The smelt is then dissolved to form green liquor, which is reacted with calcium hydroxide (Ca(OH)2) to convert sodium carbonate to sodium hydroxide (NaOH) and regenerate the original white liquor.
Methods for soapstock acidulation have been known in the art previously. U.S. patent application No. 2001/0049452 to Reaney discloses the separation and processing of a soapstock obtained from vegetable oil sources. Between 10 and 100% by weight of a monohydric alcohol is added to the soapstock. Reaney discloses that the alcohol is preferably not soluble in water, i.e. isopropanol, n-propanol, isoamyl alcohol, and fusel oil. An acid is added to acidify the soap, preferably at a pH of 4, and the alcohol layer is separated from the acid-water layer. The alcohol layer contains the fatty acids, and esterification of the fatty acids with the alcohol solvent may be carried out by heating the layer while removing the water of reaction, presumably to drive the reaction to completion.
Although there is both an acidulation reaction and an esterification reaction disclosed by Reaney, his process cannot be adapted as a single step in a Chemical mill pulping process. The alcohols disclosed by Reaney are not soluble in water and the aqueous layer is removed before heating the reaction mixture to produce the desired ester.
Acidification of tall oil soap using carbon dioxide is known in the art. PCT application No. WO 93/23132 to Huibers et al. discloses the acidulation of tall oil soap with H2SO4 to produce crude tall oil. In the process, insoluble Na2SO4 is produced.
Acid-catalyzed esterification of fatty acids with alcohols is known in the art. U.S. patent application No. 2003/0032826 to Hanna discloses a process for the production of biodiesel through the transesterification of triglycerides from animal or plant sources, with an excess of alcohol. The catalyst used may be alkali, acid, or enzyme. The reactants are injected into the reaction chamber at a temperature of between 80-200° C. Hanna discloses the use of an acid-catalyzed transesterification reaction with a triglyceride starting material, but not a free fatty acid, in order to produce biodiesel.
U.S. patent application No. 2004/0254387 to Luxem et al. discloses a method of making alkyl esters, describing a “single step” method for making biodiesel from a vegetable oil source. Luxem et al. disclose the production of biodiesel directly through the acid catalyzed reaction, without a subsequent alkali-catalyzed transesterification reaction. The reaction is performed with an excess of methanol and a sulfuric acid catalyst at temperatures between 80-200° C., and at a pressure of up to 250 psig. Luxem et al. note in a comparative example conducted at ambient pressure that the free fatty acids were completely converted into fatty acid alkyl esters, but that the glycerides were not transesterified. Luxem et al. disclose the production of biodiesel through an acid-catalyzed esterification of the starting product that includes triglycerides.
U.S. Pat. No. 4,164,506 to Kawahara et al. discloses a process for producing lower alcohol esters of fatty acids. Kawahara et al. disclose the use of the acid-catalyzed esterification of fatty acids from vegetable or animal oil sources with methanol, ethanol, or isopropanol. Methanol may be added to the fat in excess of its solubility (i.e. >12-15% by weight). The reaction is carried out between 60-120° C., but preferably at 65-70° C. to restrain inter-esterification of the fat component. The exemplary time period for the reaction is 3 hours. This reaction is followed by alkaline-catalyzed inter-esterification of the ester product. Kawahara et al. disclose a two step process, including a base-catalyzed transesterification step.
PCT application No. WO 2005/035693 to Zappi et al. discloses production of biodiesel and other valuable chemicals from waste water treatment plant sludge. Sulfuric acid-catalyzed transesterification of triglycerides and esterification of free fatty acids with alcohol is shown. The reaction is carried out at 80° C. at a pressure of 5 atm.
U.S. Pat. No. 5,008,046 to Bremus et al. discloses a process for the continuous esterification of fatty acids. The process reduces the dehydration of alkanols, especially where branched monoalkanols are used. The reaction involves the esterification of C2-C26 fatty acids with C1-C5 monoalkanols or C2-C3 dialkanols in the presence of an acid catalyst. The reaction is carried out under pressure on a reaction column at 200-900 hPa, preferably 700-900 hPa. The reactants are heated to less that 190° C., preferably 120-145° C. Bremus et al. do not disclose an acidulation step to prepare the free fatty acid.
U.S. Pat. No. 6,642,399 to Boocock discloses an acid catalyzed esterification of free fatty acids from natural sources of triglycerides with ethanol or methanol, followed by base-catalyzed transesterification of triglycerides in the mixture. The reaction is carried out at atmospheric pressure and at a temperature of 60-65° C., which is close to the boiling point of methanol. Boocock further discloses the use of a co-solvent to form a single phase solution of alcohol, fatty acids, and fatty acid triglycerides; preferably an ether. The reaction proceeds for 30-60 minutes before it is neutralized, and then the second step of base-catalyzed transesterification of the triglycerides is performed. Boocock suggests that the use of a cosolvent is a necessary component of the process and does not include an acidulation step to produce the free fatty acid, and discloses a subsequent base-catalyzed transesterification reaction.
US patent application No. 2005/0081435 to Lastella discloses an acid catalyzed transesterification of plant or animal fats with methanol to produce biodiesel. Waste oil at 50-150° F. is mixed with a catalyst and alcohol, such as sulfuric acid and methanol. The oil flows to a second reaction chamber, where a base-catalyzed transesterification reaction occurs.
U.S. Pat. No. 6,399,800 to Haas et al. discloses the preparation of soapstock, followed by the acid-catalyzed esterification of oils from plant or animal sources. Reaction conditions for the acid-catalyzed esterification are performed at 25-200° C., with a preferred temperature of 35° C. to reduce energy requirements. Pressure up to about 20 atm may be used, but atmospheric pressure is preferred. Reaction time may be as short as a half hour.
The foregoing examples of the related art and limitations related thereto are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent to those of skill in the art upon a reading of the specification and a study of the drawings.